Could Aurelian adopt Christianity instead of Sol Invictus
What would be the impact
He certainly could, although the impact probably depends largely on how long he lasts and who replaces him. IOTL Constantine was on the throne for much longer than Aurelian, and left a dynasty of Christian emperors, meaning that the notion of Christianity as the official state religion had a chance to become quite entrenched. Unless Aurelian manages to reign for longer than IOTL, his religious reforms might end up being undone by his successor.
Then again, his success in reuniting the Empire could be a considerable propaganda coup for Christianity. "The Empire was on the verge of disintegrating, then the pious Emperor Aurelian saved the day with God's help!" So if Aurelian's successor doesn't revert back to paganism and/or Aurelian himself reigns a bit longer, we might see more conversions to Christianity than IOTL.